Periodic signals, as used in animal and human communication, elicit a pitch sensation. Their spectral information is analysed in the cochlea. In addition the periodicity of acoustic signals is represented by the time course of activity in the auditory nerve. Coincidence detection of delayed and undelayed signal representations may provide an explanation for periodicity tuned responses of many auditory midbrain (IC, inferior colliculus) neurons. Correlation is high when the correlation delay is an integer multiple of the signal period. If IC neurons contribute to a correlation analysis, they should respond strongly not only for a period which fits the internal delay but also for harmonics of this period. However, most neurons in the IC show simple band-pass tuning to only one period. To explain this discrepancy, the time course of periodicity tuning and the influence of inhibition was investigated in neurons of the IC of awake gerbils. Three-hundred fourty three IC units were recorded extracellularily. In addition to the typical tuning to a pure tone (characteristic frequency), 138 of 232 units were band-pass tuned to a particular best modulation frequency (BMF) of sinusoidal amplitude modulated signals. For 23 of 28 neurons the same BMF was obtained in response to click trains or amplitude modulated white noise, indicating that periodicity tuning is independent of the waveform of the signal. In 78 of the periodicity tuned neurons (n=138) also strong reactions to the octave of their BMF were measured shortly after the onset of the stimulation. Maximal reactions to higher harmonics and sub-harmonics were also observed. Thirty to 50 ms after stimulus onset these harmonic responses were suppressed and leaving only one peak at BMF. Strong responses at harmonics of the BMF, like predicted by correlation analysis, were demonstrated in short time intervals immediately after the onset of the stimulation. Responses from 50 neurons were recorded before and during iontophoretic application of biccuculline, a competitive antagonist for GABAA, and strychnine, a competitive antagonist for glycine receptors. Blocking the inhibitory receptors of 21 units resulted in strong responses to harmonics of BMF in 6 units, even after long stimulation durations, indicating that responses to harmonics are suppressed by inhibition within the IC. A closer inspection revealed, that inhibition is coupled in phase to the modulations of the signals and acts strongest after excitatory inputs. The mean phase of modulation coupled responses was shifted by 10° on average(26 neurons). This supports the assumption that IC units receive an inhibitory input, which is phase coupled to the signal modulation and has a duration which is less than the BMF period of the units. In summary, periodicity tuning in the auditory midbrain can be explained by a superposition of a correlation analysis and a modulation low pass, which results from a delayed synchronous inhibition. This hypothesis is supported by results from simulations of networks with spiking neurons.